1. Field of the Invention
The invention relates to a catalytic cracking process for the production of good quality hydrocarbon fractions, such as gasoline with a high octane number, starting from low quality heavy hydrocarbon fractions.
2. Description of the Background
More specifically, the invention relates to a fluid catalytic cracking process in which the conversion to light fractions is carried out by putting the heavy hydrocarbon fraction in contact with a fluid bed of a particular catalytic composition.
An example of fluid catalytic cracking (FCC) is described in the U.S. Pat. Nos. 4,731,174 and 4,368,114.
This process is normally used in the petroleum industry for the conversion of heavy low quality heavy hydrocarbon fractions into high quality hydrocarbon fractions such as gasolines with a high octane number and light olefinic streams (C.sub.3 and C.sub.4).
In 1984, in the United States, the organization for environmental protection proposed for the first time, to eliminate lead from gasolines following the diffusion of catalytic converters whose catalyst proved to be irreversibly poisoned by lead.
The elimination of Pb was initially obtained in refineries by increasing the reforming capacity, introducing MTBE and improving FCC catalysts in order to optimize both the yield of gasoline and its octane number.
In spite of the use of catalytic exhaust-pipes and gasolines richer in aromatics and olefins, however, the desired improvement in the quality of the air was not reached.
It was therefore decided to introduce "reformulated gasoline" which satisfies the specifications requested by the "Clean air Act" which are the following:
at least 2.7% of oxygen; PA1 reduced level of volatile olefins; PA1 not more than 1% vol. of benzene; PA1 total aromatics not higher than 25% vol.
To satisfy these new demands, refineries directed research activities towards the study of FCC catalytic cracking processes capable of producing C.sub.3, C.sub.4 and C.sub.5 olefins with high yields. These olefins are used for the synthesis of oxygenated additives for reformulated gasoline such as MTBE, ETBE, TAME and for the production of alkylates.
In particular, research regarded catalysts which are used in FCC processes and led to the development and sale of various new catalysts.
For example, catalysts were manufactured based on zeolites stabilized with lanthanium ions which promote transfer reactions of hydrogen which cause the conversion of olefins and naphthenes into paraffins and aromatics.
The use of these catalysts however is not recommended owing to the loss of the octane number caused by the saturation of the olefins which is greater than the gain due to the formation of aromatics.
Commercial catalysts containing USY zeolites with a low content of rare earth and additives having an MFI structure, also appeared on the market.
In 1983 Mobil introduced the ZSM5 zeolite as an additive for FCC catalysts (U.S. Pat. No. 4,368,114); this additive had the characteristic of increasing the octane properties of Catanaphta and the yield of C.sub.3, C.sub.4 olefins.
ZSM 5, in fact, selectively converted the low octane components removing them from the gasoline.
As already mentioned, another way of improving the octane number of gasolines, since compounds based on lead can no longer be used, consists in adding oxygenated compounds such as MTBE and TAME.
The preparation of these oxygenated compounds, which are compatible with the environment, require the use however of large quantities of C.sub.4 and C.sub.5 olefins.